Many micro-machined electromechanical sensor (MEMS) devices and methods for manufacturing the same are generally well-known. See, for example, U.S. Pat. No. 6,642,067, entitled METHOD OF TRIMMING MICRO-MACHINED ELECTROMECHANICAL SENSORS (MEMS) DEVICES, issued Nov. 4, 2003, to Paul W. Dwyer, which is assigned to the assignee of the present application and the complete disclosure of which is incorporated herein by reference, that describes a MEMS acceleration sensor and method for manufacturing the same. In another example, U.S. Pat. No. 6,428,713, MEMS SENSOR STRUCTURE AND MICROFABRICATION PROCESS THEREFORE, issued to Christenson, et al. on Aug. 6, 2002, which is incorporated herein by reference, describes a capacitive acceleration sensor formed in a semiconductor layer as a MEMS device. Other known MEMS devices include, for example, micro-mechanical filters, pressure sensors, gyroscopes, resonators, actuators, and rate sensors, as described in U.S. Pat. No. 6,428,713.
MEMS accelerometer devices generally measure acceleration forces applied to a body by being mounted directly onto a surface of the accelerated body. One common type of MEMS accelerometer is the capacitive accelerometer. As disclosed in U.S. Pat. No. 4,435,737, LOW COST CAPACITIVE ACCELEROMETER, issued to Colton on Mar. 6, 1984, which is incorporated herein by reference, capacitive accelerometers are generally well known in the art. In a closed-loop capacitive accelerometer, the acceleration sensor is a proof mass suspended by flexures or hinges for rotation relative to an outer frame portion. The acceleration sensor is bonded between glass plates with the proof mass forming a differential capacitor with the glass plates.
The proof mass rotates about the flexures according to the principle of Newton's law: F=ma, when subjected to acceleration along the input or “sensitive” axis which is normal to the plane of the proof mass. An electrical drive and sense circuit measures applied acceleration force as a function of the displacement of the proof mass and the resulting differential capacitance.
Accelerometers of the type that are based on a rotating mass often need to be firmly constrained relative to the in-plane axes while being permitted movement in the third input axis. In devices having a rotating mass, the position of the axis of rotation also needs to be constrained. Recently, the proof mass and flexures have been fabricated in an active epitaxial or layer grown on a silicon substrate. The proof mass and flexures are structured using Reactive Ion Etching (RIE) or Deep Reactive Ion Etching (DRIE), which permits etching of very narrow slots between nearly vertical walls. DRIE permits the width, length, and thickness of the flexures to be closely controlled so that desirable bending characteristics are obtained. The flexures define a linear axis of rotation or “hinge” axis about which the proof mass moves in response to an applied force, such as the acceleration of the accelerated body, for example, a vehicle, aircraft or other accelerated body having the accelerometer mounted thereon. Traditionally, the flexures are substantially rectangularly shaped with a substantially constant cross-sectional area. The substantially rectangular shape gives the flexures greater in-plane stiffness along the major axis, i.e., the accelerometer hinge axis, and substantially less in-plane stiffness along its minor axis.
Prior art micromachined accelerometers have effectively used the substantially rectangular flexures for pliantly suspending a rotating or translating proof mass. However, the flexures of some prior art devices, such as those fabricated using RIE or DRIE, are essentially two-dimensional designs that do not permit changes in material thickness that can be used to control flexure stiffness. The rectangular flexures operate as a beam having a constant area moment of inertia, I, which is defined as the integral of the area of the cross-section times the square of the distance of the incremental area from the neutral axis. See, e.g., Shigley, MECHANICAL ENGINEERING DESIGN, 3rd edition, page 45, which is incorporated herein by reference. The rectangular flexures bend along their entire lengths, similarly to a beam of constant cross-section that is supported at both ends. The rectangular flexures therefore lack a well-defined hinge axis.